Guardian Filtration Products, Replacement Pool Spa Filter, For Pleatco PDM25P4, Unicel 4CH-21, Filbur FC-0136 COMPATIBLE WITH - Most indoor/outdoor inground and above ground swimming pools, hot tubs, and spas. EXPANSIVE FLOW CORE - Specially molded design allows higher water access to filtration fabric, reducing overall pump stress. BREAK-RESISTANT BANDING - Guardian banding prevents cartridge malfunction due to normal wear & tear. ANTIMICROBIAL END CAPS - Inhibit bacterial growth for optimal performance to deliver crystal clear water. HIGH QUALITY FILTRATION FABRIC - Engineered for durability and high dirt-holding capacity, this filter stands strong under pressure and holds up to the rigors of multiple cleanings.